1. Field of the Invention
The present invention relates to a semiconductor device, and more specifically, it relates to a hetero junction type field effect transistor which can control a short channel effect.
2. Description of the Related Art
FIG. 10 is a sectional view showing one example of a conventional HJFET (which is also called HEMT) which is disclosed in Japanese Patent Application Laid-open No. 176773/1982. The high-frequency characteristics of the HJFET are in inverse proportion to a time during which electrons flow through a gate electrode 8, and therefore, the shortening of a gate length is important to improve the high-frequency characteristics.
However, if the gate length is simply shortened, a short channel effect noticeably comes out. This short channel effect is a phenomenon that when the gate length is shortened, a threshold voltage fluctuates to a negative side and subthreshold properties deteriorate. If the threshold voltage fluctuates, the yield of elements during an element manufacture process deteriorates, and even if a gate voltage is set to the threshold voltage, a drain current begins to flow owing to the deterioration of the subthreshold properties, so that a leak current increases. Accordingly, reducing the short channel effect is desired if particularly in order to achieve a high integration, a high yield and the low consumption of electric power are required.
The above reduction of the short channel effect can be achieved, if the charge control of a channel layer 3 by the electric charges of the gate electrode 8 can be carried out more predictably.
One method, where in a conventional HJFET structure shown in FIG. 10, is to lessen a distance between the gate electrode 8 and electrons flowing through the channel layer 3 by thinning an electron feed layer 15. However, when the electron feed layer is thinned, that the threshold voltage increases, and therefore the setting of the low threshold voltage is impossible. This results because the electron feed layer decides the number of electrons in the channel layer 3 and the threshold voltage, and if the thickness of the electron feed layer decreases, than the number of the electrons present in the channel layer 3 decreases, which causes the threshold voltage to increase undesirably.
In addition, when the electron feed layer is thinned, the leak current between the gate 8 and the channel layer 3 increases noticeably. This results because, the electron feed layer 15 also functions as a barrier layer for preventing the electrons in the channel layer 3 from directly flowing into the gate electrode 8, and if the thickness of the electron feed layer 15 is decreased, the barrier layer is thinned, which causes the gate leak current to increase undesirably. Therefore, the technique of simply thinning the electron feed layer cannot be employed.
Another method comprises controlling the diffusion of the electrons which flow through the channel layer into a GaAs substrate. FIG. 11 shows a sectional structure view of an HJFET disclosed in Japanese Patent Publication (kokoku) No. 28065/1991. In the HJFET in FIG. 11, an n-GaAs layer highly doped with a donor is put in the channel layer 11, so that a band can concavely curve around the n-GaAs layer and the electrons are likely to intensively gather in the channel layer. Furthermore, this channel layer is sandwiched between i-AlGaAs layers to form hetero junction interfaces, whereby the electrons in the channel layer are prevented from diffusing into the GaAs substrate 5 and the gate. In consequence, the short channel effect when the gate length is about 1 .mu.m can be controlled. Moreover, the thickness of the channel layer 11 is 100 .ANG. or less, and hence the transportation properties of the electrons can be improved by a quantum effect, whereby the improvement of device characteristics can be accomplished.
In this HJFET structure, the short channel effect which occurs when the gate length is about 1 .mu.m can be controlled, however the short channel effect when the gate length is about 0.1 .mu.m cannot sufficiently be controlled. If the gate length is as short as about 0.1 .mu.m, the amount of the electric charges on the gate metal 8 which can be used for the control of the electric charges in the channel layer 11 decreases, so that it is difficult to control the electrons present in the channel layer 11. In consequence, the short channel effect cannot be sufficiently controlled only by preventing the electrons from diffusing into the GaAs substrate 5.
Furthermore, a second problem of the above method is that the device characteristics tend to deteriorate. This results because in the channel layer 11, the n-GaAs layer, in which the electric charges particularly intensively gather, is highly doped with the donor, and hence the transportation properties of the electrons deteriorate owing to "the ionized impurity scattering effect" caused by the donor with which the n-GaAs is highly doped.